The invention relates to an injection moulding device comprising a mould body having a cavity, an elongated nozzle seated in the cavity, a valve pin coaxially in the nozzle and actuating means connected to the valve pin for axially displacing the valve pin in the nozzle, the actuating means comprising a cilinder housing having a first pressure medium inlet connected to a first pressure medium duct and second pressure medium inlet connected to a second pressure medium duct, and a piston reciprocatable in the cylinder housing between an upper end position and a lower end position, the cylinder piston being coupled to the valve pin.
The invention also relates to a cylinder for use in such an injection moulding device.
A valve gated injection moulding device of the above-mentioned type is known from EP-A-0 836 925 in the name of the applicant. By axial displacement of the valve pin, the gate of the nozzle is opened and closed to control the flow of molten thermoplastic material into a mould cavity. The valve pin head projects from the upper part of the nozzle and is seated in a sliding bush, which is axially displaced by means of a lever arm. The lever arm is connected to a hydraulic cylinder that is placed at a radially offset location from the valve pin. In order to determine the closed and open positions of the valve pin, it is known to use external end switches that are connected to the piston, to switch off the supply of pressure medium at a predetermined stroke of the cylinder. Such construction requires a relatively large space. As the accuracy of the end switches may be temperature dependent, the known actuating cylinder and the end switches are used at a distance from the heated mould body. Furthermore, when the stroke of the cilinder is adjusted, in order to change the open and closed position of the valve pin, the end switches must be readjusted as well.
It is an object of the present invention to provide an end position indicator and/or control of the valve pin, which requires relatively little space, which is relatively insensitive to high temperatures and which allows the actuating cylinder to be used in close proximity to the valve pin head. It is another object of the present invention to provide an end position control, which is easily readjusted upon changing the stroke of the actuating cylinder. It is a further object of the present invention to provide an end position indicator/control device, which can be relatively easily installed in existing injection moulding devices.
Thereto the injection moulding device according to the invention is characterised in that the piston comprises a fluid passage via which pressure medium can pass when the piston is in a position between its upper and its lower end position, and which is at least partly closed off when the piston is in its lower and/or its upper end position.
When the piston is moving, a small flow of pressure medium from the high pressure side to the low pressure side of the cilinder occurs, via the fluid passage. This flow can be detected in the pressure medium supply ducts. When the piston reaches its end position, the opening in the piston is sealed, for instance by the cylinder housing and/or by a height adjustment element inside the cylinder housing, thus reducing the flow in the supply ducts. By detecting the flow and/or pressure in the supply ducts, it can be determined when the piston has reached its end position. By use of the end position detection according to the invention, the end switches, which terminate the pressure medium supply, need not be physically associated with the actuator cylinder, and can be placed away from the heated mould body, whereas the actuator cylinder can be directly placed on top of the valve pin head, on the manifold. As no end switches are directly connected to the actuator cylinder according to the invention, it can be of a relatively compact construction. Furthermore, no electrical supply lines for the end switches need be placed in the heated manifold area. The end position indicator according to the present invention can be easily installed in existing actuator cylinders, by providing a bore in the piston head, for allowing a small flow of pressure medium from the high pressure side to the low pressure side.
In stead of passing via a bore through the piston head, the pressure medium may also flow along the sides of the piston head, which may be in metal-to-metal contact with the cylinder housing.
In one embodiment, the cylinder comprises an internal height adjustment means, displaceable in the cylinder housing, forming a stop for the piston, for adjusting at least one of the end positions, When the stroke of the piston is adjusted by changing the position of a height adjustment ring in the cylinder housing, the end position indication will automatically be correct, and will need no recalibration.
The pressure medium can be formed by a gas, such as compressed air, but is preferably formed by hydraulic oil, which is stable at high temperatures. It was found by the applicant that the passage of oil within the cylinder from the high pressure side to the low pressure side, has a favourable effect on the service life of the oil by the reduction of degradation or xe2x80x9ccrackingxe2x80x9d or the oil.
In an embodiment, the flow through the bore in the cylinder is detected by a flow detection means in at least one of the pressure medium supply ducts, for forming a displacement signal. The displacement signal may be supplied to a control or processing means to control the operation of the injection moulding apparatus, by switching on or off the supply of molten material, or may be used to form a visual indication of the needle position, for instance using a LED indicator, or on a CRT or LCD screen. The control device and the visual indicator means can all be located outside the manifold area, without the need for electrical supply lines extending to the cilinder in said manifold area.